3Com 4200G 24-Port - MAC Address Table Management; Overview; Introduction to MAC Address Table; Introduction to MAC Address Learning

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1 MAC Address Table Management
This chapter describes the management of static, dynamic, and blackhole MAC address entries. For
information about the management of multicast MAC address entries, refer to the part related to
multicast protocol.
Overview
Introduction to MAC Address Table
An Ethernet switch is mainly used to forward packets at the data link layer, that is, transmit the packets
to the corresponding ports according to the destination MAC address of the packets. To forward packets
quickly, a switch maintains a MAC address table, which is a Layer 2 address table recording the MAC
address-to-forwarding port association. Each entry in a MAC address table contains the following fields:
z Destination MAC address
z ID of the VLAN which a port belongs to
z Forwarding egress port numbers on the local switch
When forwarding a packet, an Ethernet switch adopts one of the two forwarding methods based upon
the MAC address table entries.
z Unicast forwarding: If the destination MAC address carried in the packet is included in a MAC
address table entry, the switch forwards the packet through the forwarding egress port in the entry.
z Broadcast forwarding: If the destination MAC address carried in the packet is not included in the
MAC address table, the switch broadcasts the packet to all ports except the one receiving the
packet.
Introduction to MAC Address Learning
MAC address table entries can be updated and maintained through the following two ways:
z Manual configuration
z MAC address learning
Generally, the majority of MAC address entries are created and maintained through MAC address
learning. The following describes the MAC address learning process of a switch:
1) As shown in
Figure 1-1, User A and User B are both in VLAN 1. When User A communicates with
User B, the packet from User A needs to be transmitted to GigabitEthernet 1/0/1. At this time, the
switch records the source MAC address of the packet, that is, the address “MAC-A” of User A to the
MAC address table of the switch, forming an entry shown in
Figure 1-2.